refactor: use file structs

[tensorush]

• made use of file structs to avoid redundant namespaces
• added the simple_spacecraft_orientation to run with other examples in build.zig
• consider using doctests, I only changed the CelestialBody one to demonstrate.

[ATTron]

thanks for this! I will get to this today and look it over ⭐

[ATTron]

This all looks good to me! I might go back at some point and switch Ecs back to EquatorialCoordinateSystem, just for me I like having the full names of things when I can. If there is some zig style guide opinions about this I will defer to them, but my own personal preference is to have the full name be the structs name

[tensorush]

Sure, I think long names are fine, I changed it back. My reasoning was that it seemed the odd one out as it's comprised of 3 pretty long words, so I chose to use its acronym in code, while still having the full words in the top-level comment of its file struct definition.